1. Using brackish groundwaters as indicators for potential impacts from deeper oil and gas activities
Mark Engle, Francisco Reyes
U.S. Geological Survey
Dept. of Geologial Sciences. Univ. Texas at El Paso
El Paso, Texas

2. Acknowledgements Funding: USGS Energy Resources Program
Assistance with sampling: Madalyn Blondes (USGS), Matt Varonka (USGS), Tanya Gallegos (USGS), Josue Magana (NMSU)
Assistance with analysis: Stephanie Ray (UTEP), Belinda Gonzalez (UTEP), Jasper Konter (Univ. Hawaii), Tiffani Schell (USGS)
Thanks to the oil and gas operators whom allowed to sample their water and oil wells.

3. Potential exposure pathways to drinking water sources
Water Volume vs. Quality Impacts Mostly operation engineering problems
Modified from U.S. Environmental Protection Agency, 2015

4. Potential for upward water and hydrocarbon migration
Interception with fractures or old wells
Fluid movement may be slow (decades or longer)
Distances of several km
Difficult to assess potential
Reagan et al., 2015

5. Brackish groundwater as a proxy – Dockum Aquifer, Permian Basin, Texas
Approach: Examining brackish groundwater-brine interface
Pressure Data
Drill Stem and Initial Pad Data
Chemical Data
Na/Cl, Ca/SO4
Isotopic Data
δ18O, δ2H, 87Sr/86Sr ?
Reyes, 2014
Engle and Blondes, 2014; Engle et al., in review

6. The Permian Basin A A’ A A’
Study
Area
Major US oil producer (2012 Texas RRC data)
312 MMbbl oil
12% of US production
1,200 BCF gas A A’
Source: Texas BEG A
Triassic Dockum Group
Upper Aquifer System A’
Evaporite Confining System (ECS)
Deep Basin Aquifer System (DBBAS)
Source: UT Permian Basin

7. The Triassic Dockum Aquifer
Upper: Mud rich with thin interbedded s.s.
Lower: Sandstone rich, conglomerates, mudstones, & interbedded mud- & sandstone
Mineralogy: Qtz, calcite, alb, illite, muscovite, kaolinite
Limited uses
High salinity hazards
Natural occurring radioactivity
Used for irrigation, municipal water supply, and hydrocarbon operations.
Label graphs I’ I I’ I
Modified from Texas Water Development Board (2003)

8. 2 models for pressuring in the basin
Delaware Basin model: Over-pressuring
Palo Duro Basin model: Under-pressuring
Drill stem test data (Luo et al., 1994)
Depths of 3-5 km, gradient up to 20 kPa/m
Clay compaction disequil., dewatering, and hydrocarbon generation
Modeling work (Senger et al., 1987)
Cenozoic uplift and tilting and erosion produced substantial under-pressuring
West
East
Dockum aquifer
Modified from Matchus and Jones, 1984

9. Reservoir Pressure Gradient Data
Salt
Data for 3 counties
Data from below aquitard
Source rocks are over-pressured
Upper-most brine bearing reservoirs under-pressured since 1940s
Oil Reservoirs
Data from IHS Energy (2015) and Friedrich and Gene (2015)

10. Dockum Chemistry Overview – Durov Diagram
Na-rich waters
Higher salinity
Generally deeper
Ca-rich waters
Lower salinity
Found on basin margins

11. Water types vs. depth and location
Ca-rich waters tend to be shallower than Na-rich waters
Ca-rich waters tend to be more common in the basin margins

12. TDS ranges of the produced waters
TDS increases with depth until the Devonian
Lower TDS in the lowermost reservoirs and in the shales

13. Origin of the produced waters
Leonardian to Penn reservoirs
Evap’d Seawater
Matches data from fluid inclusions
Guadalupian and Devonian/Silurian reservoirs
Meteoric

14. Origin of the strontium in produced waters
Leonardian to Penn. Reservoir data overlap with halite + polyhalite in the evaporites

15. Dockum - Salinity Sources
Dockum waters suggest evaporite dissolution source
Brine salinity from paleoseawater and evap dissolution w/ SO4 reduction
Possibly some connection
Direction unclear
SO4 reduction

16. Dockum – Sr sources
Na-rich Dockum waters approach the composition of halite + sylvite + polyhalite
Some overlap with brines (?)
Doesn’t fit with Dockum evap dissolution model

17. Dockum - Water Sources Dockum entirely meteoric
Ca-rich vs Na-rich
Disconnect between shallow and deep brine
No evidence for upward brine migration into the Dockum

18. Conclusions
Pressure system is a hybrid of Delaware and Palo Duro models
Over-pressured source rocks
Downward pressure gradients through the evaporites
Na-rich and Ca-rich Dockum waters have different origin
Neither suggest influx from deep basinal brines
Upward flow of oil and gas related fluids from source rocks likely to be stopped in below the evaporites

19. Caveats and Final Thoughts
Patterns in vertical groundwater flow may be delayed to current conditions
Access to brackish groundwater samples is relatively rare
Pressure data for source rocks can also be difficult to obtain
But, it may have potential as a screening tool to identify relative risk from different basins

20. Conventional vs. Continuous Hydrocarbon Accumulations
Modified from Schenk and Pollastro, 2002

21. Continuous hydrocarbon accumulations
Examples
Shale Gas
Tight Gas
Tight Oil
Coalbed Methane
Characteristics
Low permeability reservoir
Pervasive across a large area, non-discrete
Not affected by hydrodynamic processes
No down-dip water contact
The Marcellus Shale, Virginia
Photo: Cathy Enomoto, USGS